The present disclosure relates to a melt-blown polypropylene sealant film for packaging applications, and especially to such a film useful as a heat seal layer of a multi-layer film that can be formed into a flexible container or retortable flexible pouch and sealed and then heated to a temperature sufficient to pasteurize or sterilize its contents.
Retort pouches come in a variety of shapes and sizes. Two common forms are the “pillow shaped” and the “gusset” or “stand-up” pouches. These pouches are considerably lighter and lower cost structures for preserving and protecting foods, condiments, medicines and sterile solutions than other structures, and are in wide spread commercial usage. Typical pouches consist of one or two sheets of laminated material which are sealed together around the periphery, generally by heat-sealing, before or after filling. In “form and fill” packaging, the pouches are formed in line by making bottom and side seals from two films brought together in surface contact with one another, adding the material to be presented, and forming the final seal to enclose the food or other substance to be packaged, all in a continuous operation. The resulting pouch is generally a pillow shaped pouch. Alternatively, the processor may employ pre-made pouches having a single open end, which are then filled and closed or sealed after filling. This technique is better suited for gusset pouches. In a final step the pouch and contents are normally heated to pasteurize, sterilize or cook the contents, such as by use of an oven or by pressurized steam in a retort. All of such structures are referred to herein as retort pouches.
In order to be used in a retort process, the pouch should comply with a number of requirements. Numerous governmental and industry requirements determine the suitability of materials which may be used for the flexible packages, especially those that will be subjected to extreme temperature environments, or contact with foods or medicines. The physical properties of these flexible pouches: lamination bond strengths, heat seal strengths, WVTR (water vapor transmission rates), OTR (oxygen transmission rates), burst analysis, opening forces, extractable content, and other properties are exacting and carefully monitored. Typically, the pouch must remain airtight and hermetically sealed after exposure to temperatures in the range of from 120-130° C. for 30 to 80 minutes at a pressure up to 500 kPa, depending on the pouch size and the contents within the pouch. In addition, subsequent usage, such as shipping, handling and dropping must not result in rupture of the sealed pouch. For some applications pouch clarity is required as well.
One type of laminate that is currently available for use in forming flexible retort pouches comprises multiple layers. One layer is designed to provide an outer protective surface to the pouch by preventing punctures or pin hole development and desirably is adapted to receive printing or graphics without migration of foreign substances such as inks or solvents to the contents of the pouch. Two or more of the foregoing polymeric layers may be provided where enhanced strength is desired. Another layer (a barrier layer) may be provided to prevent or limit water vapor or oxygen transmission to or from the pouch contents. In certain constructions, the tie layer(s) joining the various layers of the construction may also serve as barrier layers. For example, an adhesive material comprising clay nanoparticles may serve as a barrier layer in addition to or in place of a specifically designated barrier layer. The final layer is generally a sealant layer which ultimately serves to join the film (if folded upon itself) or multiple films (if the sealant layers thereof are placed in surface contact with one another) into the desired pouch through application of localized heat and pressure, such as with a heating bar or roller.
Suitable materials for forming the various layers of the foregoing laminates include polyethylene terephthalate (PET), polyamides, and polyolefins, such as polypropylene for the outer layer(s), metal foils or metal deposition layers, polyvinylidene chloride, or ethylene/vinyl alcohol (EVOH) polymers, for the barrier layer(s), and polyolefins or modified polyolefins, such as, ethylene/(meth)acrylic acid copolymers, maleic anhydride grafted polyolefins, ionomers, and blends of the foregoing with one another or with an elastomer for the sealant layer. The sealant layer may include a crystalline polypropylene (CPP), such as isotactic polypropylene, isotactic propylene/ethylene copolymers, or blends thereof with an elastomer, due to the low extractables content and good adhesive properties of such resins.
The various layers of the multilayer films may be held together by adhesives, including solvent based adhesives and, more preferably, solventless adhesives such as a curable epoxy resin or a two-component urethane adhesive. Alternatively, many constructions employ coextruded films in which the various components, optionally including tie layers or adhesive layers, are extruded from a die or a multi-block die, optionally onto a substrate comprising one or more of the layers of the resulting film, and cooled to form the desired multi-layer film. For ease of fabrication, improved performance, an ability to alternate between various materials of construction, as well as reduced component cost, or where a metal film is employed as one layer, it is highly desirable to employ preformed films for the respective polymeric or nonpolymeric layers and to construct the desired multi-layer film by adhering the various film layers to one another using an adhesive.
Suitable films, especially sealant films, for the foregoing use may be prepared by casting films having the desired physical properties. Particularly with respect to currently available polypropylene based sealant resins, the foregoing technique is necessitated because the resin employed is selected based on its adhesive properties in order to achieve a broad sealing window, and strong initial- and post-retort-adhesion properties. As a result, such resins generally are deficient in melt strength and can only be turned into films through use of cast film techniques. This is due to the fact that such resins generally will not support film formation by standard air quenched melt-blown techniques due to an inability to form a stable bubble from the molten polymer. Because the cast film process is generally a low volume/high cost process, the resulting films typically are more expensive to produce than air quenched melt-blown films (or as they are often referred to “air-quenched blown films”), thereby increasing the cost of the resulting sealant films, multi-layer films constructions, and retort pouches prepared therefrom.
It has been proposed to employ a high melt strength polypropylene for preparing air-quenched blown films and multi-layer film structures. Disadvantageously, although the resulting films possess improved toughness and appearance (especially improved hammer skin resistance) compared to a cast crystalline polypropylene films, they are generally lacking in adhesion properties, especially post-retort seal strength.
It would be desirable if there were provided a polypropylene based adhesive resin composition that is capable of use in preparing films by the air-quenched melt blown-film forming technique and which possesses suitable adhesion properties for use as a sealant layer in multi-layer retort pouch packaging applications.
It would further be desirable if there were provided a sealant film comprising a polypropylene resin that is prepared by air quenched melt blown-film forming techniques which possesses suitable adhesion properties for use in retort pouch packaging applications. Preferably, the sealant film will also contribute both stiffness and toughness (as measured by Dart impact strength) to the retort pouch.
It would also be desirable if there were provided a multi-layer laminate for use in retort pouch packaging applications, comprising as an outer layer thereof a sealant film comprising a polypropylene resin that is prepared by an air quenched blown-film forming techniques.
Further, it would be desirable for the sealant film comprising the polypropylene resin to exhibit sufficiently low extractables (both xylene soluble fraction and n-hexane soluble extractables) to be useful for environments where the sealant film may be in contact with food and other consumables, such as water.
Finally, it would be desirable if there were provided a retort pouch for use in packaging applications, said pouch being formed from one or more multi-layer laminates comprising a sealant film as an outer layer thereof, said sealant film comprising a polypropylene resin and having been prepared by an air quenched melt blown-film forming techniques.